1. Field of the Invention
The present invention relates to an optical information recording medium and a reproducing apparatus for the recording medium.
2. DESCRIPTION OF BACKGROUND INFORMATION
For the retrieval of recording information from an optical information recording medium (hereinafter, referred to as an "optical disc"), an optical system called optical pickup is widely used. A laser beam is converged to a spot-shape by an objective lens and is irradiated onto the optical disc. The irradiated laser beam is subjected to an intensity modulation by depressions called pits formed on the optical disc and the reflected light from the pit is again converged by the objective lens. The light intensity of the reflected light is photoelectrically converted by a photodetector, thereby reading of the recording information is performed.
FIG. 1 shows a construction of an optical pickup of a conventional optical disc reproducing apparatus. FIG. 2 shows a construction of a demodulating circuit.
In FIG. 1, reference numeral 51 denotes a semiconductor laser provided as a light source. Similarly reference numerals 52, 53, 54, and 55 denote a half mirror; an objective lens, a photodetector and an optical disc, respectively. In FIG. 2, reference numerals 61, 62, and 63 respectively denote a limiting circuit for converting an output signal of the photodetector 54 into a pulse train signal, a pulse width detector for measuring the pulse width of each pulse, and a data demodulator for demodulating the original digital data in accordance with a predetermined processing procedure.
As shown in an example of FIG. 3, the length of each pit which is recorded on the optical disc 55 is determined to be a value which is an integer times as long as a unit period (hereinafter, referred to as a "reference clock unit") T of a reference clock. The signal generated from the photodetector 54 of an optical pickup is sliced by the limiting circuit 61 on the basis of the center level of the amplitude and is converted into the pulse train signal. The pulse train signal is supplied to the pulse width detector 62, by which a pulse width of each pulse is detected. The data demodulator 63 demodulates the original data by examining that the pulse width corresponds to what number of times of the reference clock unit T.
The objective lens 53 of the conventional optical pickup has a circular opening. A spatial frequency transmitting characteristic (Modulation Transfer Function characteristic; hereinafter, referred to as "MTF characteristic") of the optical pickup using such a circular opening is as shown in FIG. 4. In case the conventional optical pickup is used, the following waveform distortion occurs due to imperfectness of the optical system at the i-th slice crossing point u.sub.i. EQU .DELTA.g(u.sub.i)=g(u.sub.i)-g.sub.0 (u.sub.i) (1)
where g.sub.0 represents reproduction signal of the optical system having no aberration, and g represents actual reproduction signal.
When the waveform distortion .DELTA.g(u.sub.i) occurs, a deviation in the time base directions, that is, a jitter occurs in the signal waveform. A jitter amount in this instance is obtained by the following equation. EQU .DELTA.J(u.sub.i)=-.DELTA.g(u.sub.i)/g.sub.0 '(u.sub.i) (2)
where, g.sub.0 '(u.sub.i) represents gradient at the u.sub.i position of g.sub.0.
Although the jitter occurs by the waveform distortion due to the imperfectness of the optical pickup as mentioned above, the jitter amount is inversely proportional to a gradient g.sub.0 ' as will be readily understood from the equation (2). Therefore, if the optical pickup having the MTF characteristic as shown in FIG. 4 is used and the pulse width of the output signal is judged from only the time base, the signal cannot be inserted above a relatively low spatial frequency region in which the value of the gradient g.sub.0 ' is not so small. As a recording system, further, it is necessary to set the reference clock unit T which gives the quantization step of the pulse width to T&gt;2.DELTA.J in a manner such that no reading error occurs in the demodulated signal even if a jitter is generated.
When the spatial frequency is limited as mentioned above, the kind of pit lengths which can be inserted in the limited spatial frequency region is limited and the reference clock unit T cannot also be reduced, so that a problem arises that a high density recording is difficult. For instance, in case of a CD system which is known as a compact disc, the usable spatial frequency lies within a frequency range within a frequency value about 1/2 the cut-off frequency of the modulation transfer function. As a result, only nine kinds of pits in a pit length range, from the shortest pit length 3T to the maximum pit length 11T, can be used.